Use Onward Returns on your own domain — something like returns.yourstore.com — instead of the default Onward URL.
Where to find it
Settings > Returns > Setup, under Returns General Settings.
[Screenshot: Setup tab — Custom returns portal URL with Add custom domain button]
What this is
A custom domain serves your return portal from your own branded URL. Customers stay on your domain for the entire return, and the URL is shorter and easier to share on a support call.
Setting it up
Click Add custom domain.
Enter the subdomain you want to use, for example
returns.yourstore.com.Add the DNS record shown with your domain registrar.
Wait for DNS to propagate. This can take up to 72 hours but is usually much faster.
[Screenshot: Add custom domain modal with DNS record details]
Caveats
Your default portal URL continues to work after the custom domain is active, so links you've already shared won't break.
Use a subdomain you aren't already using for something else.
If your DNS is behind a proxy such as Cloudflare, you may need to set the record to DNS-only rather than proxied.
If the domain doesn't verify after a day, check for a conflicting record on the same subdomain.
Related settings
Customer Return Portal URL — the default link, which keeps working either way